There have been known optical modulators which utilize the electrooptical effect of LiNbO.sub.3 or LiTaO.sub.3 of which the refractive index changes when an electric field is applied.
FIG. 3 shows in section a conventional optical modulator which employs LiNbO.sub.3. In the figure, an optical waveguide 2 is formed on a LiNbO.sub.3 substrate 1, a SiO.sub.2 buffer layer 3 is formed on the surface of the substrate 1, and electrodes 4, 4' are provided on the surface of the SiO.sub.2 buffer layer 3 along the optical waveguide 2 in order to apply a radio-frequency electric field onto the waveguide 2 via the buffer layer 3.
When high frequency waves, especially microwaves, are supplied between the electrodes 4, 4' from the incident end of the waveguide 2, the electrodes 4, 4' act as a transmission line to transmit the radio frequency along the optical waveguide 2. The refractive index of the waveguide 2 changes by the electric field of the radio frequency to cause phase modulation for the light which is passing through the inside thereof. If the phase velocity of the light and the phase velocity of the radio frequency can be matched (or, exactly speaking, if group velocities thereof can be matched), then the modulation bandwidth can be expanded.
It has been, however, difficult in the prior art to match the phase velocity (group velocity) of the radio frequency and the phase velocity (group velocity) of the light because the effective index for the radio frequency was high.
In the prior art, the electric field which is necessary for modulation was applied not only on the optical waveguide where it is necessary, but also unnecessarily onto other regions.
This invention aims to solve such problems encountered in the prior art and to provide an optical modulator having a wider modulation bandwidth.